1. Field of the Invention
The present invention relates to a mass flow controller and, more particularly, a mass flow controller system with diagnostic capabilities for monitoring flow conditions.
2. Description of the Prior Art
Semiconductor manufacturing apparatus is frequently supplied with various types of gas used in the manufacture of semiconductors. A mass flow controller (hereinafter referred to as an MFC) has been provided in the respective gas supply passageways to control flow rates of the respective gases. Accurate control of the gas delivery is important in maintaining production quality semiconductor elements.
FIG. 3 is a block diagram broadly showing the construction of a conventional MFC. Referring to FIG. 3, reference numeral 1 designates a supply passageway through which a fluid, such as a gas, passes. The fluid travels in the direction shown by an arrow. A flow rate sensor system 2 for detecting a flow rate of the fluid is provided on an upstream side of this passageway 1. A control valve 3 for regulating the flow rate of the fluid by changing an aperture of the valve on the basis of a valve-driving signal W from a valve-driving portion 11, which will be mentioned later, is provided on a downstream side of the passageway 1.
The flow rate sensor portion 2 comprises a flow rate sensor 4 consisting of, for example, a pair of heat sensitive sensors provided in the passageway 1 and a sensor circuit 5 that can respond to the measured heat values to calculate mass flow, so that a momentary flow rate Q of the fluid passing through the passageway 1 may be detected by means of the flow rate sensor 4 and computed by sensor circuit 5 into a detected momentary flow rate Q that is converted into an electrical flow rate sensor output signal s. Examples of mass flow controllers are found in U.S. Pat. Nos. 4,339,949, 4,685,331, and 4,947,889.
Reference numeral 6 designates a known linearizer circuit situated on an output side of the flow rate sensor portion 2. The linearizer circuit 6 is provided because the flow rate sensor output signal s is not restricted to only a proportional (linear) momentary flow rate Q, but can also be subject to a nonlinear momentary flow rate Q. The flow rate sensor output signal s can be compensated, for example, by a polygonal line approximation, followed by outputting a momentary detected flow rate Q.sub.S of the fluid as a flow rate measuring signal.
The momentary detected flow rate Q.sub.S is provided to an analog output terminal 7 so as to be displayed in a flow rate display, and can simultaneously be applied to a comparison (error-detecting) portion 8. An analog input terminal 9 is connected with this comparison portion 8 so that a flow rate-setting signal S.sub.Q for setting a desirable flow rate may be input through this analog input terminal 9. The comparison portion 8 is adapted to output a differential signal .epsilon.(=S.sub.Q -Q.sub.S) between the desired flow rate-setting signal S.sub.Q and the momentary detected flow rate Q.sub.S by comparing the flow rate-setting signal S.sub.Q with the momentary detected flow rate Q.sub.S.
Reference numeral 10 designates an operational control portion carrying out a PID control and the like and adapted to carry out an appointed operation by, for example, implementing the following equation using the differential signal .epsilon. from the comparison portion 8 and preliminarily set control constants, such as P, I, and D (Proportional, Integral, and Derivative), to thereby output the resulting control signal V. EQU V=.epsilon..multidot.P+I.intg..multidot.dt+D(d .epsilon./dt)
That is to say, in the case where .epsilon.&lt;0 holds good, the control signal driving the control valve in a closing direction is provided, while, in the case where .epsilon.&gt;0 holds good, a control signal driving the control valve 3 in an opening direction is provided.
Reference numeral 11 designates a valve-driving portion adapted to apply a valve-driving signal W to the control valve 3 on the basis of the control signal V from the operational control portion 10.
Accordingly, with the MFC having the above-described construction, the momentary detected flow rate Q.sub.S from the flow rate sensor portion 2 is compared with the flow rate-setting signal S.sub.Q in the comparison portion 8, and the opening of the control valve 3 can be controlled on the basis of the control signal V obtained as a result of the differential signal provided from the comparison portion 8, whereby the flow rate of the fluid can be controlled.
However, in the above-described conventional MFC, the operational condition can be established only from the flow rate-setting signal S.sub.Q supplied to the MFC from an outside source and the measured momentary detected flow rate Q.sub.S, but the actual flow cannot be detected until the flow really occurs; that is, until the difference between the flow rate-setting signal S.sub.Q and the momentary detected flow rate Q.sub.S really occurs.
Accordingly, a preexisting problem cannot be found until it affects the flow rate. This can be a serious problem in that a large quantity of articles of inferior quality could be produced or the manufacturing line could be suspended for a long time period in the manufacturing process of semiconductors and the like.
In addition, in the conventional MFC having the above-described construction, the detecting sensitivity of the flow rate sensor 4 and the P, I, and D control constants are subtly changed depending upon the type of fluid, the actual flow rate, and the like, so that, in the case where a different fluid is used and the flow rate is changed after it is incorporated into the manufacturing line of semiconductors, the MFC must be removed from the manufacturing line of semiconductors and then readjusted or exchanged with another MFC. This takes much time and labor and, thus, the manufacture of semiconductors is delayed.
Furthermore, in the conventional MFC, the flow rate measuring signal s from the flow rate sensor portion 2, the flow rate signal output to the linearizer circuit 6 provided between the flow rate sensor portion 2 and the comparison portion 8, the control signal V from the operational control portion 10 and the like have not been able to be read out from the MFC, so that any abnormalities in the flow rate sensor portion 2, the linearizer circuit 6, and the operational control portion 10 have been unable to be detected on an individual basis.
Thus, there is a need in the prior art to provide an improved MFC.